•In
4thweek,
a thickening of the mesenchyme occurs on the side of the embryo in the
frontonasal prominence = lens placode =area where eyes develop that begins on the sides of the
head(eyes will rotate to front eventually)

•A
pair of thickenings on the front of the frontonasal prominence = nasal
placodes = where nose will form

•maxillary
prominence is paired but unlike the mandibular prominence, is not fused.(Eventually they fuse with medial nasal prominence.If this doesn’t happenàcleft
lip)

How is the gap between the maixillary prominences formed?

§No
gap exists before this stage—only a groove exists between the maxillary
prominences and frontonasal prominences = stomodeum

§This
groove deepens and approaches the front of the digestive tract, since at
this point there is no connection between the front of the digestive tract
and outside world
§The
deepening of the stomodeum continues until only a thin film remains between
the outside world and the digestive tract = oropharyngeal membrane
§Eventually
the oropharyngeal membrane will rupture and create an opening between the
maxillary prominences = primitive mouth (allows the pharynx to
communicate with amniotic cavity= “outside world”)

At 5 weeks…

•Eyes
move to front of head

•There
is a proliferation of mesenchyme that forms the nasal placodes

•Nasal
placodes differentiate into a lateral nasal prominence and a
medial nasal prominence

•The
lateral nasal prominence will become the alae of the nose

•A
depression develops in the center of the nasal placodes = nasal pit

§The
oronasal membrane eventually ruptures so the nasal cavity is almost
completely confluent with the oral cavity.A small piece of intermaxillary segment at the very front of the
developing mouth remains as the only separation between them.

§

How do the oral and nasal cavities separate?a.k.aHow does the
palate form?

Remember…at ~6 wks there is no separation…you get separation during
the 7thweek:

§The
nasal septum forms from a fold/sheet that begins at the top of the
nasal cavity (in the midline) and grows down to divide the nasal cavity
into left and right halves
§Lateral
palatine processes are horizontal folds (also running front to back)
that develop from the sides and grow medially.They meet and fuse at the midline along with the nasal septum.

§-->
so now we have two separate right and left nasal cavities
which are themselves separate from the oral cavity

§the
tongue is beginning to develop at the bottom of the oral cavity

§The
lateral palatine processes (LPP) can be seen growing in from the
sides (toward eachother) as well as forward to meet the IMP

§The
palate is formed by this convergence of IMP and LPP’s and has
2 parts:

•Primary
palate = part formed by the IMP

•Secondary
palate = part formed by the LPP’s

§Primary
palate will meet secondary palate → secondary palate will begin to
fuse from front to back

§Incisive
foramen =where primary
and secondary palates meet

§Closure
occurs by the 12thweek(but it is all soft tissue)…

Which bones develop from the secondary palate?

•Maxilla
–bears the molars, premolars and canines; not the
incisors.

§The
intermaxillary process (IMP) is located at the front of
future mouth

•Palatine
bone forms behind the maxilla

The back of the secondary
palate does not ossify, giving rise to a dip at the end
called the uvula.

bones from the primary palate?

1.Premaxilla
(premaxillary part of the maxilla)—bears the incisors.

So why don’t all the ossified tissues of the palate make up a
single bone?

****Because
the embryonic origins of the maxilla and premaxilla are
different!****

The maxilla comes from the maxillary prominences (from
the 1stbranchial)

arch

The premaxilla comes from the intermaxillary segment
(from the frontonasal

prominence)

As a result, the premaxilla is considered a separate bone
that fuses with the maxilla.